738 THE NERVOUS SYSTEM 



nerve-cells lies at the base of the anterior cornu externally; it is limited to the 

 dorsal region, but cells occupying relatively similar positions are found in the 

 cervical and luml)ar regions also. The axons of its cells probably constitute the 

 katalwlic or motor nerve-tibres of the muscles of the glandular and vascular 

 systems. 



A middle column of cells can be recognised in the centre of the grey matter 

 at the junction of the anterior and posterior cornua. It is more or less distinct 

 throughout the whole length of the cord, and its function is unknown. 



The term processus reticularis is applied to a number of strands of grey 

 matter which project into the white matter and anastomose together on the outer 

 side of the base of the posterior cornu. 



The central canal of the cord is lined by columnar epithelium; it is continued 

 into the tilum terminale for aljout half the length of the latter, and is here some- 

 Avhat dilated; a few nerve-tibres accompany the canal into the filum terminale, and 

 are supposed to be rudimentary coccygeal nerves. The canal is dilated in the 

 conus medullaris. forming an irregular cavity, the sinus rhomboidalis inferior, 

 or ventriculus terminalis. 



lu transverse sections through the spinal cord made in the dissecting room the general 

 arrangement of the grey and white matter can be made out, and sections from the different 

 regions of the cord can be distinguished from one another in fresh specimens, especiallj' with the 

 aid of a pocket lens. 



In the upper cervical region (at the level of the second or third cervical nerves) the grey 

 matter forms an H-shaped outline bearing a great resemblance to the outline in the dorsal region 

 (fig. 439, B). Sections through this region are very difficult to distinguish from sections of the 

 dorsal cord with the naked eye. The microscope, however, reveals the absence of Clarke's 

 column and the presence, in some cases, of root-fibres of the spinal accessory nerve. 



In the lower ceiTical region (fig. 439, A) the cord is elliptical in outline and the anterior 

 horns of the grey crescents are very large ; the section shows a certain similaritj' to a section 

 through the lumbar region (fig. 439, C). In the latter, however, the anterior horns are more 

 evenly rounded, the general outline approaches more nearly to a circle, and the proportion of 

 grey to white matter has gi'eatly increased. 



Sections of the dorsal region can be easily recognised with the microscope by the presence of 

 Clarke's column, which is confined to the dorsal, lowest part of the cervical, and the uppermost 

 part of the lumbar regions. 



Sections through the part of the cord from which the sacral nerves arise show a great jjre- 

 ponderance of grey matter. This feature becomes more marked as the termination of the cord is 

 approached (fig. 439, D, E, and F). 



THE DEEP CONNECTIONS AND ASSOCIATIONS OF THE SPINAL 



NERVES 



But little information can be gained from a mere macroscopical or microscopical 

 examination of the adult cord concerning the course of the fibres of the nerve-roots 

 within it, and the facts detailed here have been chiefly ascertained by embry ©logical 

 and i)athol()gical research. The success of the embryological method depends 

 upon the fact tliat certain tracts or groups of fibres myelinate (or acc[uire their 

 medullary shi'ath) at an earlier period than others, and therefore these tracts of 

 nicdullated lilires can be followed and recognised through a series of sections. The 

 pathological method rests upon the influence Avhich nerve-cells exert upon nerve- 

 tilires; this so-called trophic influence depends upon the fact that every nerve-fibre 

 is a ])rocess of a nerve-cell and can only exist and retain its functions as long as it 

 remains connected with the cell. The trophic cell, therefore, of every nerve-fibre 

 is that cell from which the fibre arises and with which it is in direct structural con- 

 tinuity. If the trophic cell is destroyed the nerve-fil)re dies, or if the structural 

 continuity of the cell and fibre is interrupted l)y section, then the fibre degenerates 

 and dies beyond the point of interruption, and the degeneration and death occur 

 sinudtaneously throughout tlie whole length of the separated portion. In connec- 

 tion also with the tro})hic infiucnce of nerve-cells it must be borne in mind that 

 every nerve-cell with its processes constitutes a distinct entity, called a neuron, 

 which has close associative relations with other nerve-cells and with various tissues 

 of the body by means of its processes, but it is absolutely devoid of structural 



